A bidirectional voltage-regulator diode has an NPN (PNP) junction and has characteristics that a breakdown occurs when a reverse bias voltage is applied to PN junctions connected in series in mutually opposite directions and a current flows in both directions at a fixed voltage (breakdown voltage).
However, the so-called snap-back effect occurs in the bidirectional voltage-regulator diode by which the breakdown voltage drops together with the current rise under the influence of transistor operation caused by the NPN junction, creating a problem that a high breakdown voltage cannot be obtained.
When the snap-back effect occurs, the breakdown voltage is not determined by the withstand voltage of a PN junction and a lower voltage than the withstand voltage of the PN junction shows up, which is caused by a current that flows when a breakdown occurs as a base current of the transistor structure.
A planar bidirectional voltage-regulator diode that reduces the current amplification factor when an N layer in the NPN structure is considered as a base layer of a bipolar transistor is known to control the transistor operation by the NPN junction.
To reduce the current amplification factor, methods such as introducing impurities to be a life time killer into the P layer and increasing impurity concentrations partially in the P layer are known. However, there are problems of more complex manufacturing processes of elements and an increased element area involved in an element structure. Thus, there is a need for a bidirectional voltage-regulator diode that can be manufactured more easily by controlling the transistor operation by the NPN junction.